As is well known, photomasks employed in the large-scale manufacture of semiconductor devices and integrated circuits may take the form of transparencies which contain regular arrays of normally identical optical elements each representative of an individual device or circuit. Typically, each element on a mask is a photographic image produced by optical reduction from a master image in a conventional "step and repeat" operation.
Evidently, any defects in the images on the mask will lead to corresponding defects in the associated devices or circuits fabricated with the use of the mask. Generally, such mask defects are of a nonperiodic nature, wherein defects appearing in a single element of the array have no discernible relation to any other element of the array. To avoid the manufacture of defective devices and circuits of this nature, it is important that all the images on the associated mask be examined for such errors before the mask is employed in production. Unfortunately, visual inspection of the mask, even with the aid of a microscope, projector, or similar device, is time consuming and tedious; this is particularly true when, as in the usual case, the mask may contain hundreds of closely spaced, minute elements. The same drawback is present, of course, when inspecting other types of two-dimensional patterns containing regular arrays of elements, such as grids for television-augmented telephone sets supplied under the name PICTUREPHONE by Western Electric Company, Inc.